This invention relates to a pulse width control circuit, and more particularly to a pulse width control circuit of the type which controls the output pulse width by means of a feedback loop.
Exemplarily, an ignition unit for an internal combustion engine employs a pulse width control circuit. Typically, the internal combustion engine ignition unit has a signal generator, built in a distributor, which produces a signal output synchronized with the rotation of the engine. The signal output is then processed by an amplifier having a waveform shaper and a driver circuit, which drives an output transistor loaded with an ignition coil to generate a high voltage across a secondary winding of the ignition coil. This high voltage is applied via the distributor to an ignition plug for firing the same.
In recent years, such ignition devices are required to produce increasingly higher outputs for the purpose of meeting stringent exhaust gas controls and improving the efficiency of fuel combustion. For production of high outputs, there may be available a method which is directed to increasing the magnitude of electric current I.sub.L1 which flows through the primary winding of the ignition coil. Here, electric current I.sub.L1 and energy E.sub.L1 on the primary side of the ignition coil can be expressed as follows: EQU I.sub.L1 =(V.sub.cc /R.sub.L1).multidot.(1-e.sup.- (R.sub.L1/L1)t) (1) EQU E.sub.L1 =1/2.sub.L1 .multidot.I.sub.L1.sup.2 ( 2)
wherein,
V.sub.cc denotes the power source voltage,
L.sub.1 denotes the inductance on the primary side of the ignition coil,
R.sub.L1 denotes the resistance on the primary side of the ignition coil,
t denotes time, and
e denotes natural logarithm.
From the foregoing formulas, it is self-evident that an increase in electric current I.sub.L1, on the primary side of the ignition coil, is effective for increasing the output of the ignition unit. The signals to be generated by the signal generator built in the distributor have periods varying in a very wide range of 500 ms to 5 ms, for example, and in this range of signal periods, sufficient energy must be constantly supplied to the primary side of the ignition coil. Thus, it becomes necessary to decrease the time constant by decreasing the inductance on the primary side. When the time constant is decreased, however, the electric current I.sub.L1, as is noted from formular (1), flows excessively when the engine is operated at a medium to low speed, although the magnitude of the electric current is proper and moderate at a high speed of the engine operation. To avoid this inconvenience, when reaching a prescribed magnitude, the electric current needs to be suppressed so that this prescribed magnitude may be kept. For a successful increase of the output of the ignition unit, therefore, it is necessary to adopt a method which effects the current suppression in the range of low to medium engine speed.
In this current suppression, collector voltage V.sub.c of the output transistor under the current suppression condition is given by the following equation: EQU P.sub.c =V.sub.cc -I.sub.L1 .multidot.R.sub.L1 ( 3)
Let P.sub.c stand for electric power consumed by the output transistor under the current suppression condition, and the following equation will be satisfied. EQU T.sub.c =(V.sub.cc -I.sub.L1 .multidot.R.sub.L1).multidot.I.sub.L1 .multidot.D (4)
where, D denotes a percentage of time for current suppression within the signal period.
From the equation (4), it is seen that, in the current suppression method, the electric power P.sub.c consumed by the output transistor increases in proportion as the power source voltage V.sub.cc increases because the electric current I.sub.L1 on the primary side of the ignition coil is fixed to the prescribed value. This fact implies that if the amplifier of the ignition unit is constructed in the form of a semiconductor integrated circuit, having an amplifier chip, an output transistor chip and passive elements such as capacitors and resistors unified in the form of a thick-film hybrid integrated circuit on a single substrate, then temperatures of junctions in the semiconductor integrated circuit will be greatly affected by the electric power consumed by the output transistor. Particularly, it is very difficult to maintain the junction temperature constantly below the maximum allowable level under the conditions of high power source voltage and high temperature because thermal design for thermal resistance of the substrate between the output transistor chip and the thick-film hybrid integrated circuit faces extreme difficulties when taking the harsh environmental conditions inevitably encountered by the ignition device of the internal combustion engine.